1. Field of the Invention
The present invention generally relates to a self-light-emitting device (also referred to as an EL device). In particular, the present invention relates to such a self-light emitting device in which an EL element, which is constructed of an anode, a cathode, and a light emitting organic material (hereinafter, referred to as organic EL material) with which EL (electro luminescence) is obtained, is sandwiched therebetween, is formed on an insulator, and to a method of manufacturing electric equipment having the self-light-emitting device as a display portion (display or display monitor). Note that in this specification, a description will be made of an EL display device as the above stated self-light-emitting device.
2. Description of the Related Art
In recent years, the development of display devices using an EL element (EL display device) as a self-light-emitting element which utilizes the EL phenomenon of light emitting organic material has been advancing. The EL display device is a self-light-emitting device, and therefore it doesn""t need a back light such as that of a liquid crystal display device. In addition, the EL display device has a wide angle of view. As a result, the EL display device is looked upon as promising as a display portion of electric equipment.
EL display devices are classified into two: a passive type (simple matrix type); and an active type (active matrix type), both of which have been actively developed. Particularly, the active matrix EL display device is attracting attention these days. With regard to organic EL materials to be an EL layer which can be said to be the center of an EL element, low molecular weight organic EL materials and high molecular (polymer) organic EL materials have been studied. The low molecular weight organic EL materials are formed by vapor deposition or the like, while the high molecular organic EL materials are formed through a coating using a spinner.
With respect to both the low molecular weight organic EL material and the high molecular (polymer) organic EL materials, when the surface on which the EL material is formed is not planarized, there is a problem in that the thickness of the formed EL material can not be even.
Further, in case that the thickness of the EL layer is not even and the EL layer is partly not formed at a step portion, when an EL element formed of a cathode, the EL layer, and an anode is formed, the cathode and the anode are short-circuited.
When the cathode and the anode are short-circuited, electric current intensively flows between the cathode and the anode, and almost no electric current flows through the EL layer, which makes the EL layer not to emit light.
The present invention has been made in view of the above, and an object of the present invention is to improve the structure of an EL element, and to provide a method of manufacturing an EL display device. Moreover, another object of the present invention is to provide an electric equipment having such an EL display device as a display portion.
In order to attain the above objects, according to the present invention, there is employed a structure such that when an EL layer is formed by an organic EL material for forming the EL layer, an insulator is embedded to planarize an uneven portion on the surface where the organic EL material is to be formed, thereby preventing a short circuit between a cathode and an anode in an EL element from occurring. FIGS. 1A to 1C illustrate the structure of cross sections of a pixel portion of an EL display device according to the present invention.
FIG. 1A illustrates a TFT for controlling electric current, which is electrically connected to a pixel electrode 40. After a base film 12 is formed on a substrate 11, the TFT for controlling electric current is formed so as to have an active layer including a source region 31, a drain region 32, and a channel forming region 34, a gate insulating film 18, a gate electrode 35, a first interlayer insulating film 20, a source wiring 36, and a drain wiring 37. Note that, although the gate electrode 35 is of a single-gate structure in the figure, it may be of a multi-gate structure.
Then, a first passivation film 38 is formed at the thickness of 10 nm to 1 xcexcm (preferably 200 to 500 nm). As the material, an insulating film containing silicon (especially, a silicon oxynitride film or a silicon nitride film is preferable) can be used.
A second interlayer insulating film (which may also be referred to as planarizing film) 39 is formed on the first passivation film 38 so as to cover the respective TFTs to planarize a step formed by the TFTs. As the second interlayer insulating film 39, an organic resin film such as a polyimide resin, a polyamide resin, an acrylic resin, or a resin containing a high molecular compound of siloxane is preferable. Of course, an inorganic film may also be used if it can perform sufficient planarization.
It is quite important to planarize, by the second interlayer insulating film 39, a step formed by the TFTs. Since an EL layer to be formed later is very thin, existence of a step may cause failure light emission. Therefore, it is preferable that planarization is performed prior to the formation of the pixel electrode in order to make as planar as possible the surface on which the EL layer is formed.
Further, reference numeral 40 denotes a pixel electrode (corresponding to an anode of the EL element) formed of a transparent conductive film, and is formed so as to be connected to the drain wiring 37 of the TFT for controlling electric current through a contact hole (opening) which is formed in the second interlayer insulating film 39 and the first passivation film 38.
According to the present invention, as the pixel electrode, a conductive film formed of a compound of indium oxide and tin oxide is used. A small amount of gallium may be doped into the compound. Moreover, a compound of indium oxide and zinc oxide, or a compound of zinc oxide and gallium oxide may be used.
Note that a concave portion 46, formed after the pixel electrode is formed in the contact hole, is herein referred to as an electrode hole. After the pixel electrode is formed, an EL material is formed to form an EL layer. In this case, however, as shown in FIG. 1B, the thickness of the EL layer in the electrode hole 46 becomes thinner in thin film region 47. Though the extent of the thinning of the film thickness depends on the tapered angle of the electrode hole, among the film forming surfaces, portions which are not vertical with respect to the film forming direction tend to have difficulty in having the formed film and tend to have thinner film thickness.
However, if the formed EL layer becomes thinner here, and in addition, a disconnected portion is formed, the cathode and the anode in the EL element are short-circuited, and electric current intensively flows through this short-circuited portion. This prevents electric current from flowing through the EL layer, which makes the EL layer not to emit light.
Accordingly, in order to prevent the short circuit between the cathode and the anode in the EL element, an organic resin film is formed on the pixel electrode so as to sufficiently fill up the electrode hole 46. By patterning the formed organic resin film, a protective portion 41b is formed. In other words, the protective portion 41b is formed so as to fill up the electrode hole. Note that a similar protective portion (not shown) of an organic resin film may also be formed in a space between pixel electrodes so as to fill up the space.
The organic resin film is formed by spin coating. After exposing the organic resin film to light using a resist mask, etching is performed to form the protective portion 41b as illustrated in FIG. 1C.
Note that the thickness of a rising portion in cross section of the protective portion 41b from the pixel electrode (a portion illustrated as Da in FIG. 1C) is 0.1 to 1 xcexcm, preferably 0.1 to 0.5 xcexcm, more preferably 0.1 to 0.3 xcexcm.
Also, the material of the protective portion 41b is preferably an organic resin such as a polyimide resin, a polyamide resin, an acrylic resin, or a resin containing a high molecular compound of siloxane. Further, the viscosity of such an organic resin used is preferably 10xe2x88x923 Paxc2x7s to 10xe2x88x921 Paxc2x7s.
After the protective portion 41b is formed, as illustrated in FIG. 1C, an EL layer 42 is formed, and further, a cathode 43 is formed. Note that the EL material forming the EL layer 42 may be a low molecular weight organic EL material and may be a high molecular organic EL material.
By forming the structure illustrated in FIG. 1C as in the above, the problem of the short circuit between the pixel electrode 40 and the cathode 43 caused when the EL layer 42 is disconnected at a step portion in the electrode hole 46 can be solved.